1. Field of the Invention
This invention relates to so called crossed field mass spectrometric analyzers and ionizers which separate ionized gas particles according to their mass to charge ratio by motion in an electric field and a magnetic field perpendicular to each other.
2. Description of the Prior Art
The use of mass spectrometers in determining the identity and quantity of constituent materials in a gaseous, liquid or solid specimen has long been known. It has been known, in connection with such systems, to analyze the specimen under vacuum through conversion of the molecules or atoms into an ionic form, separating the ions by mass to charge ratio, and permitting the ions to bombard a detector. See, generally, U.S. Pat. Nos. 2,882,410; 3,070,951; 3,590,243; and 4,298,795. See, also, U.S. Pat. Nos. 4,882,485 and 4,952,802.
In general, mass spectrometers contain an ionizer inlet assembly wherein the specimen to be analyzed is received, a high vacuum chamber which cooperates with the ionizer inlet, an analyzer assembly which is disposed within the high vacuum chamber and is adapted to receive ions from the ionizer. Detector means are employed in making a determination as to the constituent components of the specimen employing mass to charge ratio as a distinguishing characteristic. By one of many known means, the molecules or atoms of the gaseous specimen contained in the ionizer are converted into ions, which are analyzed by such equipment.
It has been known with prior art cycloidal mass spectrometers to use a fixed collector and ramped electric field in looking at only one mass to charge ratio at a time. In many prior art mass spectrometer systems, regardless of whether they were of the cycloidal type or not, the ionizers were quite large and, as a result, dominated the design and specifications of the systems to be employed therewith.
U.S. Pat. No. 5,304,799 discloses a cycloidal mass spectrometer having a housing defining an ion trajectory volume, an electric field generator for establishing an electric field within the ion trajectory volume and an ionizer for receiving gaseous specimens to be analyzed and converting the same into ions, which travel through orthogonal electric and magnetic fields and subsequently impinge on a collector. This spectrometer was designed to have a plurality of different ions"" mass to charge ratios impinging on the collector depending on the strength of the fields. It was stated that the cycloidal mass spectrometer and ionizer may be miniaturized to as provide a small, readily portable instrument.
It has been known to employ crossed fields mass spectrometry in two types of analytical problems. It has been employed in the identification of molecules with high molecular weight. It has also been employed in precise measurement of the relative abundance of isotopes.
It has also been known to employ mass spectrometers in connection with situations involving low mass to charge ratio such as in helium leak detectors and hydrogen analyzers. Mass spectrometry has been employed in such situations as it is nearly free of interference within the mass range and because of its sensitivity. The analyzers typically employed in helium leak detectors, for example, are generally smaller copies of larger analyzers such as sector field mass spectrometers which are easier to manufacture but provide a lower performance level and tend to be relatively expensive.
Quadrupole analyzers are small and less expensive than magnetic separators, but their filter quality decreases when approaching the lower end of the mass scale. The so-called xe2x80x9czero blastxe2x80x9d represents the contribution of the particles the quadrupole is not tuned on as a result of the weak filter characteristics. For a helium leak detector, for example, the zero blast portion of hydrogen interferes with the helium signal at about 4 amu.
The present invention focuses on field structures of a cycloidal mass spectrometer wherein a circular motion is imposed by a linear motion.
In one embodiment of the invention which may function as a mass spectrometer or analyzer, first and second planar, generally parallel electrodes which generate an electrical field therebetween and have projecting walls which cooperate with the base of the general planar electrodes to define an ion generating chamber. Electric fields generated by the electrodes are oriented perpendicular with respect to a magnetic field which may be generated by permanent magnets or electromagnets in a manner well known to those skilled in the art. Certain ion beams based upon mass to charge ratio exit through an ion exit with the other separated ion beams being separated on the mass to charge basis being retained within the ion generating chamber. An ion collector operatively associated with the ion exit is positioned adjacent thereto and may cooperate with processing means well known to those skilled in the art in determining the identity of the molecule or atom. Employing the same apparatus without the ion collector can result in the device functioning as a mass selective ion generator. Related methods are also disclosed.
The apparatus and method of this embodiment are particularly structured to be employed with low mass materials which may be 20 amu or less.
It is an object of the present invention to provide a mass spectrometer which is particularly suited to the special requirements of the low mass range.
It is a further object of the present invention to provide such a mass spectrometer wherein the electric field while not necessarily uniform serves to separate the trajectories of ions of different mass number at the low end of the mass scale, rather than providing high resolving power at the upper end of the mass scale.
It is a further object of the present invention to provide such a mass spectrometer wherein the real focusing properties can be replaced by approximation achieved by designing special field profiles in three spatial dimensions.
It is yet another object of the invention to provide such a mass spectrometer wherein the electrodes would be of small dimension, of simple structure and inexpensive to manufacture.
It is yet another object of the invention to provide an analyzer or ionizer which will be robust against imperfections in the magnetic field without materially interfering with the desired results and thereby allowing the use of small, inexpensive magnets.
These and other objects of the invention will be more fully understood from the following detailed description of the invention on reference to the illustrations appended hereto.